Long Term Evolution (LTE) is a standard within the 3GPP (3rd Generation Partnership Project) with an aim to improve the UMTS (Universal Mobile Telecommunications System) mobile phone standard for coping with future technology evolutions.
In LTE, a physical uplink control channel (PUCCH) is used for conveying uplink control information, such as for example channel quality indication (CQI) and uplink scheduling requests. A user equipment (UE) has two ways of notifying a scheduler in a base station (denoted evolved Node B or eNB in LTE) that is has data to transmit. The UE may have a PUCCH scheduling resource for scheduling requests (SR), and in such case it simply uses this PUCCH scheduling resource to send the scheduling request to the base station thus notifying that it has data to transmit. The scheduling request can be seen as a simple flag (single bit) and there is no need to provide the identity of the UE since this is implicitly known from the identity of the PUCCH scheduling resource on which the scheduling request was sent. The second way for a UE to send a scheduling request, if the UE does not have a PUCCH scheduling resource, is to instead perform a random access (RA) procedure.
The 3GPP LTE standard thus supports the UE having a PUCCH scheduling resource or the UE using a RA procedure for the scheduling request. However, in order for all users to have a scheduling resource it is necessary to use a large portion of the available physical resource blocks (PRB) for PUCCH, which limits the number of PRBs that are available for traffic data.
There are also drawbacks with the use of RA procedure for the UE to notify the scheduler of the base station that it has data to send. Firstly, the use of RA procedure entails more messaging between the UE and the base station, which increases the delay before the UE is scheduled for data transmission. Secondly, the processing load in the base station for handling the RA procedures may be higher than for handling scheduling requests on PUCCH. Using RA procedures thus means that the base station would have to be optimized for a higher RA load, which entails higher costs for processing capacity.
In practice, the amount of available scheduling resources limits the number of users that can be handled in a communication system with low delay.
Further, since the scheduling resources form a bottleneck, the base station needs to reserve scheduling resources for prioritized UEs, e.g. emergency calls. This further adds to the complexity of the scheduling resource handling functionality of the base station.
From the above it is clear that the PUCCH resources are limited and used at the expense of traffic resources, and that the alternative, using RA procedures, entails longer delays for the user before being granted access. It is clear that there is a need for an improvement on this situation in this field of technology.